The present invention relates to a high-withstand-voltage and large-current semiconductor device, and particularly relates to an improvement of a power semiconductor device in which gate is driven by a voltage and high speed switching operation with realization of a low on-voltage.
As So a device configuration having such a high-withstand-voltage and large-current capacity (hereinafter referred to as "power semiconductor device"), a bipolar transistor, a power MOSFET and so on have been well known. On the other hand, effective use of advantages of these power semiconductor devices is made in, a high-speed and low-on-voltage insulated gate type bipolar transistor (hereinafter abbreviated to "IGBT") which has attracted keen interest, and this IGBT is one type of power semiconductor devices which have been growing rapidly recently.
FIG. 5 shows a sectional structure of an IGBT having a conventional general configuration which is illustrated typically, and FIG. 6 shows an equivalent circuit thereof.
That is, in the sectional structure in FIG. 5 and the equivalent circuit in FIG. 6, this IGBT is constituted by a MOSFET 21 and a PNP transistor 22. The MOSFET 21 is constituted by an n.sup.- -type base region 5 as a second conductivity type base region, a p-type well 6 as a first conductivity type base region formed on the n.sup.- -type base region 5, and an n.sup.+ -type source region 8 selectively formed by diffusion in the p-type well 6, and the PNP transistor 22 is constituted by an p.sup.+ -type silicon substrate 7 as a first conductivity type collector region, the n.sup.- -type base region 5 as the second conductivity type base region, and the p-type well 6 as the first conductivity type base region. The reference numerals 1 to 3 represent an emitter (E), a gate (G) and a collector (C) respectively, 4 represents a gate polysilicon layer (gate electrode) forming the gate 2 through a gate insulating film 10, and 9a and 9b represent metal electrode layers (emitter electrode and collector electrode) forming the emitter 1 and the collector 3 in the same manner. The reference numeral 11 represents an n-type buffer layer.
As is apparent from the equivalent circuit in FIG. 6, a current passed through the MOSFET 21 constituted by the n.sup.- -type base region 5, the p-type well 6 and the n.sup.- -type source region 8 in the structure in FIG. 5 becomes a base current of the PNP transistor 22 constituted by the p.sup.+ -type silicon substrate 7, the n.sup.- -type base region 5 and the p-type well 6, for turning on the PNP transistor 22. That is, in this IGBT, turning-on can be realized not by supplying a current but by applying a voltage to the gate 2. Thus in an IGBT, gate driving can be realized by a voltage, and particularly with respect to its on-voltage, the on-voltage can be set to a comparatively low value unlike the case of a normal power MOSFET, because the IGBT has a characteristic of a bipolar type device represented by the PNP transistor 22.
As an improved configuration obtained by modifying the IGBT having the above-mentioned structure of FIG. 5, a device having a sectional structure shown in FIG. 7 is disclosed in "Proceedings of 1990 International Symposium on Power Semiconductor Device & ICs Tokyo", pp. 117-121. FIG. 8 is an equivalent circuit of the same device.
In the sectional structure of FIG. 7 and the equivalent circuit of FIG. 8, parts which are the same as those of the abovementioned IGBT shown in FIGS. 5 and 6 are referenced with its same reference characters. In addition, the reference numeral 20 represents an n.sup.+ -type drain region selectively formed by diffusion in the p-type well 6 in which an n.sup.+ -type source region 8 is similarly formed. The FIG. 7 structure also includes an NPN transistor 23 and a base short-circuit resistance 24.
The conventional power semiconductor device shown in FIGS. 7 and 8 is different from the above-mentioned general IGBT shown in FIGS. 5 and 6 in that an output current is made to flow by an npnp thyristor of four layers constituted by the n.sup.+ -type drain region 20, the p-type well 6, an n.sup.- -type base region 5 and a p.sup.+ -type silicon substrate 7, so that it can be considered that the on-voltage is low. The on-voltage can be made substantially lower in the case of the power semiconductor device of FIGS. 7 and 8 than in the case of the IGBT of FIGS. 5 and 6.
However, in the case of the conventional power semiconductor device having the above-mentioned configuration shown in FIGS. 7 and 8, there has been various problems as follow.
That is, as is apparent from the equivalent circuit of FIG. 8, in the thyristor having the above-mentioned configuration, the base-emitter circuit is shorted by the base short-circuit resistance 24, and the thyristor is maintained in its off state normally, so that even if the MOSFET 21 constituted by the n.sup.+ -type drain region 20, the p-type well 6 and the n.sup.+ -type source region 8 is brought into its on state, a current hardly flows while its collector voltage is low because the thyristor is in its off state as mentioned above.
As the collector voltage increases gradually so that a depletion layer is widened in the p-type well 6, the base short-circuit resistance 24 also gradually increases. This is because the base short-circuit resistance 24 is formed of the p-type well 6a and the current path becomes narrow as the depletion layer becomes wide.
Even if the thyristor is in its off state in this state, the value of the current increases little by little by a recombination current in the depletion layer with the increase of the collector voltage, so that a potential drop is caused across the base short-circuit resistance 24 in the p-type well 6. When this potential drop increases sufficiently to make the bipolar type NPN transistor 23 conductive, an amplification factor a.sub.1 of the NPN transistor 23 and an amplification factor a.sub.2 of the PNP transistor 22 satisfy the relation a.sub.1 +a.sub.2 &gt;1 so that the thyristor goes into its on state.
FIG. 9 shows a current-to-voltage characteristic in the conventional power transistor of FIGS. 7 and 8. In FIG. 9, Vi represents a voltage for the above-mentioned thyristor to reach its on state, that is, a so-called breakover trigger voltage.
As shown in FIG. 9, for the above-mentioned reason, there is a negative resistance region in the thyristor. Accordingly, in the conventional power semiconductor device shown in FIGS. 7 and 8, the thyristor returns to its off state again, even at its conductive time, in such a low current band that the sum of the above-mentioned amplification factors a.sub.1 +a.sub.2 in the thyristor portion is smaller than 1. Thus, it is very difficult to use the power semiconductor device.
AS has been described, indeed, the thyristor in nature realizes a low on-voltage by positive feedback by current carriers in the two bipolar type transistors 22 and 23, but if the thyristor has substantial voltage discontinuity at its on time as shown in FIG. 9, that is, a negative resistance region, and if the scattering of the voltage vi to reach the on state exists in a chip, on-operation is performed only in a portion where the voltage Vi herein is low, so that current concentration is caused in this corresponding portion.
Indeed such current concentration in the corresponding portion is not an important problem in practice in the case of a small-capacity semiconductor device having a comparatively small chip size, but in the case of a large-capacity power semiconductor device having a large chip size, the effect can easily be so large as to cause device breakdown in the corresponding current-concentration portion.